As you might know already – I seriously love rock climbing. And many other scientists that come to Antarctica share this passion. But a dead-flat ice shelf is probably the worst place to be a climber… so how do we keep up our strength in Antarctica?
Well, you have to be creative because there is no branch to do your pull up workout. Luckily Bruce and I had a fantastic idea and recycled our snow pit that was used for firn density measurements. Within our little rock climbing gym, we work not only on our pull up strength. But we also refran from building stairs out of it. So how do we get out of the snow pit? Hopefully with a muscle up by the end of the field work… and if not, we are probably still stuck in a snow pit.
What is the coolest thing in Antarctica? Hot-water drilling ! Because once the hole is finished, there is time to celebrate with style. But let me tell you more about it.
Bruce and I share a common love for digging in the snow. But even the keenest of snow diggers won’t get much deeper than a couple of meters below the surface. How can we get even deeper and all the way through the hundreds of meters of ice beneath our feet ?
We use hot water to drill a hole through the floating ice to gain access to the ocean underneath. Once the hole is finished we drop a sediment corer down through the hole, and pull an approximately 1.5 m long sediment core from the ocean floor. The core tells us when exactly the grounding line has retreated at this location – because sediments floating in the ocean waters can only settle if there is no ice. After we have pulled the core, we use two cameras that look up and downwards to film layers within our borehole and especially how the underside of the ice shelf looks like. Are there any rocks frozen into the ice (upward looking camera)? Or are there flourishing ecosystems on the ocean floor as described by Jules Verne’s science-fiction novel ‘10000 leagues under the sea’ (downward looking camera)? In our final step, we then deploy oceanic instruments through the hole to measure water temperature/salinity and the strength of the current. Additionally a series of thermistors are placed into the hole – all while racing against the time until the hole freezes again. So how long does it take to drill a hole?
Martin and Dale as the drilling specialists in our team say “it takes us about 2 days to prepare the drill, then 1 day to melt the snow for the drilling water, and once we have started drilling about 5 hours for a 400 m deep hole.” But then there is the magic moment when they melt their way through the last inch of ice and the hot water in the borehole escapes into the ocean cavity underneath. “We then have about 24 hours to do all the experiments before the hole freezes again.” I’m impressed, this means that they don’t get any proper sleep in several days ! Absolute heroes… So what do you do after everything is finished? “Well, there is still a lot of hot water left in our tanks – so we soak in the hot tub and enjoy the view.” I seriously can’t think of anything cooler than sitting in a hot tub on a glacier in Antarctica !
Today we explore the unknown lands of Antarctica. While we can map Antarctica’s surface with satellites, our knowledge of what lies beneath the ice is very limited. Radar can be used to estimate what Antarctica’s bedrock looks like and has revealed surprising results. For example, the South Pole’s surface elevation is 2835 m (9333 ft) and it sits on about 3000 m of ice. This means, that the weight of the overlaying ice is pushing the bedrock below sea level. So let’s cut a transect through Antarctica and have a closer look!
Our transect starts far away in the open ocean. Here, research vessels have been used in the past to map the ocean floor with sonar. These measurements show that the deep seabed goes down to below -4000 m beneath the ocean surface. As our transect reaches West Antarctica’s continental shelf, the ocean floor jumps up rapidly to -500 m and remains at this depth until we reach the West Antarctic Ice Sheet. The ice itself is heavy, and pushes the ocean floor back down. It is here where the ice is getting thicker towards the South Pole and the bedrock is sloping inland causing the West Antarctic Ice Sheet to be inherently unstable. Closer to the South Pole, the Transantarctic Mountains cause the bedrock to rise again which acts stabilizing. So where is the problem?
It is the areas where we can’t map the ocean floor from research vessels or airborne radar. This is because sea ice prevents ships to access certain areas, or because the ocean surface is covered by a 400 m thick floating ice shelf as for our two field sites. In these areas, active seismics is the only way to accurately map the ocean floor and pinpoint canyons in the continental shelf that allow warm ocean waters to access the West Antarctic Ice Sheet and cause rapid melting. What is active seismics? Now this will blow your mind – it means to use dynamite on the ice-shelves surface.
The seismic signals of the explosion travel through the ice shelf, are then reflected by the ice base and can be captured again at the surface. But some of the seismic signal even travels through the ocean waters underneath the ice, are then reflected by the ocean floor and captured by the same geophones on the surface. Atsu, as our seismologist, is then using these measurements to map the ocean floor underneath the ice shelf.
These maps then help us to answer fundamental questions of ocean circulation in this critical region. When did the ice thin to a point that our Dotson Camp site became ungrounded? This thinning would have made Bear Peninsula to a Bear Island, which allowed warm ocean waters to access parts of the ice shelf that is wasn’t able to erode before. Today we have made our first successful seismic experiment, and we are about to find out when!
Today we say goodbye to Cavity Camp on the Thwaites Glacier and move onto the Dotson Ice Shelf. We are very interested in this particular area, because several ‘ice rises’ have here become ungrounded in the last decade. Ice rises are high points in the ocean floor (like underwater mountains) on which the ice can rest. The additional drag from resting on these pinning points provides additional stability to the feeder glaciers (good). However, our satellite measurements show that all ice rises on the Dotson Ice Shelf become smaller and some have even detached already (bad). This is evident in the map, where grounding line rings from the early 1990s (blue) have become smaller over the millennium (yellow lines) and almost disappeared in 2014 (red). So why have they detached?
The reason is most likely, that the ice has thinned so much from basal melting, that it is not thick enough anymore to rest on the tops of the underwater mountains. Our measurements will (a) show that this is the explanation for the disappearance of pinning points, and (b) highlight areas of rapid basal melt. So how do we measure basal melt in the field from the surface of the ice shelf?
With an Auto-phase sensitive radio echo sounder (or short, the ApRES). This state-of-the-art instrument has been developed by the British Antarctic Survey and is our Starship Enterprise in the fleet of glaciological instruments. What makes it so special? The instrument can measure ice thickness at its location to millimeter accuracy. This means that if we take a measurement and return after one week, we can measure the change in ice thickness at this location. But here is an example:
Let’s say we take a measurement today (light blue curve in the graph) and the depth of the ice base below the surface is 650.42 m. We then return after one week and take the measurement again on exactly the same location and it is 650.0 m (yellow curve). This means that 42 centimeters will have melted away in 7 days at this location – this corresponds to a rate of 25 m (82 ft) per year ! Holy penguin.
But the scientist in you says that 7 days isn’t long enough to represent an entire year, right? For this reason we first measure as many of these points as possible (light blue circles in the map) and then repeat them after 7 days. We can now tell the areas with high from the areas with low basal melt. We then modify the ApRES from attended mode to unattended mode and leave the instrument sitting in the field – continuously transmitting its measurements via satellite link to my warm office in Oregon. With this time series we can now tell if our map of basal melt rates from the repeat measurements is representative, or if there are seasonal trends of increased basal melting. I’m particularly interested if the basal melt is connected to ocean tides which might play an important role in this crucial area.
phote courtesy of Daniel Price
What lies beneath our feet? Measuring the ice thickness and quantifying the amount of accumulation is essential for glaciological research. But how do we do that? A common method consists of pulling a radar behind a skidoo on the ice surface or hauling the system by foot.
Ground penetrating radar (GPR) is a useful tool for mapping of features hidden under the ice surface. As ice is very transparent to radar signals, the signal can penetrate deep from the transmitting antenna into floating glaciers and ice shelves. Layers within the ice then act like a mirror and reflect parts of the signal back to the receiving antenna on the surface. We then record how much time it took between sending it out and receiving the signal – the so-called ‘two-way travel time’. From the lab we know how quickly radar signals travel through ice, so we can easily convert the two-way travel time directly to depth of the reflecting layer below the surface. But here is an example:
From the gym we know that Usain Bolt travels with 10.44 meters per second through air. If we let him race for 4022 s, he would have run 42 kilometers (that’s a marathon in 67 minutes). And now you say that he will get tired and can’t keep his 10.44 m/s up for that long. So how long can he keep it up? His world record over 200 m is 19.19 seconds – that is only 2.003 times his time over 100 m. So he can keep it up for at least 200 meters ! Anyway, you get the point. If you know the travel velocity of Usain (or radar) through a medum (like ice) and you stop the time, you can calculate the overall distance. But there is one mistake that can happen even to the best radar-glaciologist… the measured ice is suddenly twice a thick as expected, why ? Because you still have to divide the two-way travel time by a factor of 2.
But as easy as it sounds, there is a bit more to it. Different radar waves can penetrate to different depths – with lower frequencies penetrating ice thicknesses up to several kilometers. With these low-frequency radar systems we can then see what lies beneath the ice. Is it grounded on bedrock or floating on the ocean? Are there basal channels at the ice base through which meltwater is discharged? Or are there any crevasses at the bottom of the ice? Higher frequency radar systems don’t penetrate all the way to the ice base as their signals are reflected from internal layers near the surface. With these systems we can measure how thick eventual snow bridges are over burried crevasses or if there are any spatial differences in snow accumulation. Sometimes we see very clear internal layers and need to have an even closer look. We can then either drill an icecore to retrieve a sample, or if it is closer to the surface we do it the old way and grab a shovel and start digging. If there is one thing I have learned in Antarctica, it is digging.
Unfortunately though, radar isn’t perfect… as useful as it is for measuring internal layers and ice thickness, radar doesn’t tell us anything about the ocean floor underneath. Other techniques, such as (a) gravimetry measuring spatial differences of the strength of the gravitational field, and (b) active seismology using seismic waves from controlled explosions, can be used to map the ocean floor.
This woman crush Wednesday goes out to Tina Lutz. We have met a few years ago when I was testing a weather station in our backyard in New Zealand. Tina was travelling around the globe and came to visit my flatmates Sam’n Rohan in Christchurch. We got along very well, but she had to continue her travels to OZ and I deployed to Antarctica. After a suspiciously long hug, Tina gave me this sticker of her Bingo Club with the words: ‘Put it somewhere epic, Chriz !’
In the meantime I’ve been to the icy continent several times and still couldn’t decide on the best place for it… but on the plus side, I’m stoked to call Tina my girlfriend today.
This Wednesday marks 6 weeks I’ve been in Antarctica and is officially half way through our trip. I thank her for incredible support and am looking very much forward to seeing her soon in New Zealand where our story began. Much Aroha
After snowfall bound us to our tents for the last three days, clear blue skies and crisp conditions allowed the first field measurements on the Thwaites Glacier today. The ice is generally thinner than we have estimated from satellite data before coming to Antarctica. This is important because it indicates a change in the structural stability of this part of Thwaites Glacier. Underneath our camp, ice thickness is just about 300m, followed by 550m of ocean water to the sea floor.
Due to the good flight conditions this evening we were also visited by a BBC crew. They are filming a new documentary about climate change in Antarctica and were really impressed by our beautiful Cavity Camp. As this is a collaboration between the US and the UK, we offered our new English friends a warm cup of tea – with water freshly melted from snow, and full cream milk powder to add these extra calories to keep them warm.
We are all very much looking forward to a busy time over the holidays as the weather forecast for Antarctica promises a White Christmas… Hohoho
Stay cool everyone,